Computer display systems have advanced a tremendous amount since the days of the simple cathode-ray tube monitor. New flat panel displays have a myriad of capabilities and can support a wide range of resolutions and refresh rates. They are being driven by signals compliant with new standards, such as DisplayPort, and other new standards that are currently being developed or will be developed are sure to follow.
Often a new host computer, such as a laptop or desktop computer, having these advanced capabilities needs to communicate with an older display, referred to as a legacy display. These legacy displays may be Video Graphics Array (VGA) or Digital Visual Interface (DVI) displays.
A DisplayPort device communicates device information over a differential auxiliary channel, while a VGA device communicates device information over an I2C bus. An adapter can be used to translate DisplayPort signals used by a host to VGA or DVI signals used by these legacy displays. This means the adapter needs to be able to receive a DisplayPort instruction from the host and translate it into an I2C compatible instruction, as well as receive I2C data and translate it into DisplayPort data for the host.
There are at least two aspects to the timing involved in translating DisplayPort auxiliary signals to VGA or DVI I2C signals. First, since the host acts as the master, it issues instructions at a certain rate. These instructions are translated by the adapter and passed to the display, which is the slave. If these instructions issue at a rate that is incompatible with I2C display circuitry, the host may not be able to retrieve needed data from the display. Second, the bus speeds between the DisplayPort auxiliary channel and the I2C bus are often incompatible. Typically, the DisplayPort auxiliary channel speeds are higher than the I2C bus speed. If the adapter provides instructions using an incompatible bus rate, the host may not be able to retrieve needed data from the display.
Thus, what is needed are circuits, methods, and apparatus that allow a DisplayPort compatible host device to control data transactions over an I2C bus when communicating with a legacy monitor.